Rectified alternating current type coded track circuit signaling system



March 1952 w. H. REICHARD RECTIFIED ALTERNATING CURRENT TYPE CODED TRACK CIRCUIT SIGNALING SYSTEM 2 SHEETS-SHEET 1 Filed Oct. 19, 1945 H s ON xu Kukm INVENTOR.

Hl5 ATTORNEY March 4, 1952 w REICHARD 2,588,044

RECTIFIED ALTERNATING CURRENT TYPE CODED TRACK CIRCUIT SIGNALING SYSTEM Fled Oct. 19, 1945 2 SHEETS-SHEEZT 2 E BY M 41 H15 ATTORNEY Patented Mar. 4, 1952 RECTIFIED ALTERNATING CURRENT TYPE CODED TRACK CIRCUIT SIGNALING SYS- Wade H. Reichard, Rochester, N. Y., assignor to General Railway Signal Company, Rochester,

Application October 19, 1945, Serial No; 623,220

9. Claims.

1 This invention relates to coded track circuits for railroads and it more particularly pertains to improvements in the transmission and reception 1 of code pulses at the opposite ends of a track section.

In the conventional type of coded track circuits most commonly employed in practice where track circuit codes are transmitted in both directions through the track rails, a code transmitter relay at each end of the track section operates its contacts at a predetermined code rate to connect alternately a source of energy and a code following track relay across the track rails at that end of the track section. This is done to alternately transmit and receive driven and inverse or off-code pulses at each end of the track section. The contact that interrupts the track relay circuit breaks a circuit having a light current load which is generally fixed at only the amount of current required to operate the relay, but the contact that interrupts the track circuit feed, breaks a circuit which may at times have a very heavy load because of the shunting of the track rails by a train or because of low ballast resistance. In order to break these circuits which are of low voltage and high current, silver contacts on the code transmitting relays are generally required because of their low resistance and high thermal conductivity. Because of the softness, low melting point, and low boiling point of silver, the contacts wear enough to require frequent replacement-of the code transmitter relays.

An object of the present invention is to reduce the amount of current that will be required to be broken by the contacts of the code transmitter relays. This is accomplished by feeding the track circuit from a source of alternating current, through step-down transformers and rectiers, and inserting the contacts of the code transmitter relays in the high-side of the step-down transformers. In this manner the current required to be broken by the code transmitter relay contacts is reduced from the current of the conventional circuit arrangement by the ratio of the step-down transformer.

Another object of the present invention is to provide a means by which code transmitter relays having contacts harder than silver, such as tungsten, can be employed to further prolong the life of the code transmitter relays. Tungsten contacts have the advantage of being harder than silver and have higher melting and boiling points, but their use directly in the low voltage track feeds in theconventional manner is prohibitive because of their relatively high resistance. The high contact resistance at the surface of the contacts in the low voltage track feeds would cause a variable voltage drop in the track circuit that would make it diiiicult to regulate the voltage applied across the track rails within the required operating margins. The variable voltage drop across the contacts when used in the high-side of the step-down transformers, however, is reduced in its effect upon the low voltage track circuit feed by the ratio of the step-down transformer to a point where a variable voltage drop across the tungsten contacts in the high-side of the transformers will not materially affect the :3 regulation of the voltage at the track rails.

Another object of the present invention is to provide for a reliable operation of a polarized relay of the magnetic-stick type for the reception of inverse or off-codes, without the use of a code transmitter contact as a commutating contact in its circuit. It is desirable to use a magneticstick relay for receiving the inverse code pulses because it is possible to eifectively lengthen the cod pulses for decoding purposes. Shorter pulses such as the inverse code pulses actually transmitted through the track rails are more diflicult to decode.

Other objects, purposes and characteristic features of the present invention will be in part obvious from the accompanying drawings and in part pointed out as the description of the invention progresses.

In describing the invention in detail, reference will be made to the accompanying drawings in which those parts having similar features and functions are designated by like letter reference characters which are generally made distinctive by reasons of preceding numerals indicative of the location of the signal with which such parts are associated, and in which:

Fig. 1 illustrates a typical coded track circuit for a stretch of track signalled for east-bound traffic as one embodiment of the present invention; and

Fig. 2 illustrates a modification of the typical track circuit shown in Fig. 1.

The illustrations employed in the disclosure of this embodiment of the present invention have been arranged to facilitate the disclosure as to the mode of operation and the principles involved, rather than for the purpose of illustrating the construction and specific arrangement of parts that would be employed in practice. Thus, the relays and the contacts are shown in a conventional manner.

which these symbols are used always have cur rent flowing in the same direction. The symbols BX and CX are employed to indicate relative instantaneous positive and negative connections respectively to a suitable source of alternating current.

In order to simplify the description of the present invention, reference will be made from time to time to functions common to all parts of a similar character by use in the description of letter reference characters without their preceding numerals. It is to be understood that such reference applies to any partsv designated in the drawings by reference characters that are similar except for numerals preceding the letters.

The trackway for which this embodiment of the I present invention is provided comprises a main stretch of track divided into track sections in the usual manner and having wayside signals associated therewith. The signals shown are of the color light type having individual red, yellow and green color light units, but it is to be understood that other types of signals such as search light, semaphore, or positionlight signals could as Well be employed, and the system could be used with cab signals with or without wayside signals. I

Associated with the track section illustrated in Fig. l is a track relay 2TR connected across the track rails through a suitable adjustable resistor. Such track relay is of a polarized type having a structure to cause it to be responsive only to energization with a particular polarity. The type of relay generally employed is of the two-position biased polar type, and it can be operated away from its deenergized position only by the energization of its winding with a particular polarity.

, There is a track feed at each end of the track section comprising a source of alternating current connected across the track rails through a step-down transformer 'IFR, a rectifier, and suitable adjustable resistance.

At the right hand end of the track section, a magnetic-stick type approach relay BAR is connected across the track rai1s.,for thereception of inverse code impulses. Such relay is of a two-position polarized type. The contacts of such relay will remain in their last operated position until the energization of the relay with the opposite polarity. The means for applying different polarities to the relay in this embodiment of the present invention is accomplished by the selective energization of two separate coil wind-= ings which are connected in opposition to each other.

' Decoding circuits are provided at the ends to codes of a particular rate such as the rate of 180 pulses per minute. Similarly, the approach repeater relay 3ARP is energized in response to. the reception of code pulses by its associated relay 3AR.

At the right hand end of the track section,'a,

code repeater relay 3GP is provided for transmitting the driven code pulses, and it is selectively energized to transmit at a 180 or '75 rate in accordance with its connection to the coding contacts [C or 15C of a suitable code oscillator or other type of impulse forming means.

At the left hand end of the track section the inverse code transmitter relay 21M? is provided for the transmission of inverse code pulses as shown in Fig. 1, and the relays ZTPA and ZTPB are provided in the modified form shown in Fig. 2 for a similar purpose.

To consider more specifically the circuit organization for the track circuit, assume that the trackway is unoccupied by a train. Under such conditions a driven code is transmitted from the right-hand end of the track section and an inverse code is transmitted from the lefthand end.

With reference to Fig. 1, a source of alternating current at volts or other suitable line voltage is fed to the primary winding 20 of transformer 3TFR by the closure of the circuit for such primary winding at front contact 2| each time the relay 3GP is picked up for the transmission of a code pulse.

the track relay ZTR, through the adjustable resistance 4|. More specifically, at the same time that the relay 2TB. is energized during a driven code pulse, a circuit is closed from the upper terminal of winding 22 of transformer 3'I'FR, through the rectifier unit 23, upper rail of track section 2T adjustable resistance 4i, rectifier 3B,

and lower rail of track section 2T, to the lower,

terminal of winding 22 of transformer 3TFR. The winding of relay 2TB is of sufficiently low resistance to provide that it will operate properly irrespective of the energization of the multiple circuit described above.

In response to the picking up of relay 2TB, the shifting of contact 25 of such relay causes an induced voltage to be transmitted through the decoding transformer 26 and the rectifying contact 21 of relay 2TR to the winding of relay 2H. Relay 2H is maintained steadily picked up byits slow acting characteristics between code pulses as long as code pulses are received by the relay ZTR, irrespective of whether such pulses are received at a 75 or a 180 rate.

Under the assumed conditions of unoccupancy of the trackway, the relay 3H is picked up in accordance with the energization of a track circuit (not shown) for track section 3T corresponding to the manner in which relay 2H is picked up in response to the transmission of a driven code in track section 2T. In accordance If the relay 3H were to be dropped away, as bythe presence of a train in advance of signal 3,. the shifting of contact 28 would .be effective. to 1 open the circuit for contact I80C andclosea: circuit for contact in. an obvious manner.

When the voltage is induced in the secondary winding 22 of the trans- In accordance with the transmission of a 180 Code through the track circuit, the relay 2D is picked up because of the energization of its circuit which is tuned to a 180 rate. Such circuit comprises winding 29 of transformer 26, condenser 38, decoding transformer 3| and rectifier 32.

The making and breaking of contact of relay 2TB causes a voltage to be induced in the secondary winding of transformer 33 for the energization of the inverse code transmitter relay ZIMP. The presence of the rectifier unit 34 in the circuit for relay ZIMP, however, causes such relay to be responsive only to the breaking of front contact 25 of. clay 2TB, thus causing relay 2IMP to be pick up only upon the termination of each driven code pulse received over the track circuit.

Each time the inverse code transmitter relay 2IMP is picked up, an obvious circuit is closed for the energization of the primary winding 35 of transformer ZTFR from the source of alternating current having its terminal connections designated as BX and CX upon the closure of contact 33.

The voltage induced in the secondary winding 3'! of transformer ZTFR is rectified by the bridge type rectifier 38 to provide for energization of the track circuit with full-wave rectified energy by a circuit extending from the positive terminal of the rectifier 38 through the lower rail of the track section ET, the rectifier unit 39, the lower winding of relay SAR, adjustable resistor 48, upper rail of track section 21', and adjustable resistor 4| to the negative terminal of the rectifier 38.

At the same time a multiple circuit is closed through the track feed winding 22 of the transformer 3TFR and rectifier 23, but the resistance of the lower winding of relay 3AR is such as to allow suilicient current to flow through such winding to provide for proper operation irrespective of the multiple path through the winding 22 of the transformer STFR. Such multiple circuit extends from the positive terminal of rectifier 38 including the lower rail of track section 2T, winding 22 of transformer S'IFR, rectifier unit 23, upper rail of track section 2'1, and adjustable resistance 4| to the negative terminal of rectifier 38.

Upon the energization of relay 3AR by the circuit just described, the contacts 8! and 42 of that relay are operated to their left-hand positions to cause the relay SARP to be energized by a pulse set up in the decoding transformer-44 in a manner similar to that which has been described more in detail for the energization of the relay 2H through the decoding transformer 26.

The inverse code transmitter relay 2IMP is only momentarily picked up because of the collapse of the flux in the primary winding of transformer 33, and the opening of contact 35 of relay ZIMP terminates the inverse code pulse being transmitted. At the right-hand end of the track section, however, the relay 3AR, because of being a magnetic stick relay, is maintained in its lefthand position until it is oppositely positioned upon the beginning of the next driven code pulse. This arrangement effectively prolongs the inverse code pulse as far as the decoding by the transformer 44 and the relay 3ARP is concerned.

The beginning of the next driven code pulse is eifective to cause the relay 3AR to be energizzd with a polarity to operate its contacts BI and 42 to their right-hand positions by a circuit closed. from the lower terminal of the secondary winding 22 of transformer STFR, including the lower rail of track section 2T, winding of relay Z'IR, variable resistor 24, upper rail of track section 2T, adjustable resistor 45, and upper winding of relay 3AR to the upper terminal of the primary winding 22 of transformer 3TFR. In actual practice a large percentage of the current for the energization of the upper winding of relay 3AR is ballast leakage current, and therefore bypasses the winding of relay 2TB, which is included in the circuit described above. The upper winding of relay 3AR is shunted by the rectifier unit 23, and therefore causes the energization of the circuit for the upper winding of relay 3AR to be principally the opposite half of the alternating current wave induced in the winding 22 from the half of the wave which is applied to the track section ET for the transmission of the driven code pulse. It will be noted that a rectifier unit could be inserted in series with the upper winding of relay 3AR, and poled to pass only the half of the wave required for the operation of such relay if desired, but it has been found that such rectifier is not needed if proper impedance is provided for the winding of relay 3AR as compared to that for the recti her 23.

There is also another circuit that is closed through the winding of relay SAR during the transmission of a driven code pulse. Such circuit extends from the lower terminal of winding 22 of transformer BTFR including rectifier unit 39, lower winding of relay 3AR, adjustable resistor 0, adjustable resistor 45, and the upper winding of relay SAP. to the upper terminal of winding 22 of transformer 3TFR. Energy flowing through this circuit flows through both windings which are connected in opposition, and therefore the energization of this circuit has little if any effect upon the relay EAR. The more direct energization of the upper winding as has been described, is effective to overpower any unbalanced erlectrof energization that may be set up by the energization of both coils connected in series in the manner just described.

Because relay 3ARP is picked up under normal conditions with the trackway unoccupied, the obvious circuit for the control of the red, yellow and green lamps of signal 3 is held open at back contact 46. If an eastbound train were to enter the track section 2T, however, the shunting of the track rails would be effective in an obvious manner to cause the relay 3AR to cease operating its contacts BI and 42 to their left-hand positions, and thereby cause the relay 3ARP to be dropped away and close the back contact 46 to provide for the energization of the lamps R, Y and G of signal 3 as selected by the contacts 47 and 48 of relays 3H and 3D in accordance with the condition of occupancy of the trackway in advance of that signal.

The selective energization of the relays 3H and 3D in accordance with the condition of occupancy of the trackway in advance of signal 3 is effective in a manner comparable to the manner shown for the control of relays 2H and 2D at the left-hand end of the track section 2T. It has been shown that the relay 2H is maintained picked up whenever a code of either a 75 or a rate is received, and therefore is energized whenever the track section 2T is unoccupied by a train. The relay 2D, however, is responsive only to a code at a 180 rate, and such code is provided for transmission by the relay 3H only when the track 7 section 3T is unoccupied by a train. The selective energization of the red, yellow and green lamps of signal 2 is therefore provided in an obvious manner by contacts 49 and so in accordance with the selective energization of the relays 2D and 2H respectively.

With reference to the circuit organization described, it will be noted that two components of current flow through the track rails. One component is that provided through the halfwave' rectifier 23, and this component supplies the principal part of the track circuit current. The other component is alternating current in character that passes through the upper winding of the relay 3AR. It has been pointed out that because of the rectifier 23 being connected in multiple with this winding, the energization of such winding is more strongly provided by the opposite half of the alternating current wave from that half used for the main track circuit feed. The impedance of this winding of the relay 3AR is relatively high as compared to that of the rectifier 23 so as to cause the AC component applied to the track rail through the winding of the relay 3AR to be at a lower voltage than the rectified energy applied through the rectifier 23. In accordance with this arrangement, the higher voltage impressed upon the track circuit by the track feed through the rectifier 23 is effective to cause the polar track relay Z'IR at the opposite end of the track section to be responsive to such energization, although the alternating current component of the current nowing throu h the track circuit also fiows through the winumg of sucn track relay.

Another embodiment of the present invention is shown in Fig. 2 in which parts corresponding to parts shown in Fig. 1 are given corresponding reference characters. In this embodiment the transmission and reception of driven codes is eiiected in the same manner and by the same circuits as has been fully described with reierence to Fig. 1, so reference can be made to the description as it has been set forth with respect to the embodiment shown in Fig, l for a description of these circuits.

For the transmission of an inverse code, according to the system shown in Fig. 2, the relay ZTPB is picked up during the reception or" a driven code pulse at the left-hand end or the track sec-' tion 2T by the energization of a circuit extending from including front contact 65 of relay 2H, back contact 5| of relay ZTPA and winding of relay ZTPB to Upon termination of the driven code pulse, the relay 2TH. is dropped away and the closure or back contact 25 of such relay causes relay ZTPA to be picked up by the energization of a circuit extending from including back contact 25 of relay 2TH, front contact 52 of relay 2H and winding of relay ZTPA to The picking up of relay ZTPA, with relay ZTPB already picked up, closes a circuit for the primary winding 35 of transformer ZTFR extending from BX including front contact 53 of relay zTrB, front contact 54 of relay ZTPA and winding 35 of transformer ZTFR, to OK.

The energization of the primary winding 35 of transformer ZTFR causes the energization of the track circuit, for the track section 2T to provide for the energization of the upper winding of relay 3AR by a circuit extending from the positive terminal of the full wave rectifier 38 including the lower rail of track section 2T, upper winding of relay 3AR, back contact 55 of relay 3GP, upper 8 rail of track section 2T and adjustable resistor 4| to the negative terminal of rectifier 38. It has been shown that the transmission of the pulse is dependent upon the picking up of relay ZTPA upon termination of a driven code pulse, so therefore the code transmitter 3GP at the right-hand end of the track section 2T is deenergized, and

contact is closed at that time. The energization of relay 3AR in this manner causes the operation of the contacts 6| and 42 of such relay to provide for the energization of relay 3ARP in a manner corresponding to that which has been discussed with reference to Fig. 1. contact 55 of relay 3GP in Fig. 2 is comparable to the rectifier 39 in Fig. 1 that it prevents the energization' of the uppefwinding of relay SAR during the transmission of driven pulses.

It will be noted that the picking up of relay ETPA opens the circuit for relay 2TPB at back contact 5|, but the rectifier unit 51 shunted across the winding of relay ZTPB, through the adjustable resistor 58, causes relay ZTPB to be sufiiciently slow in dropping away to provide as long an inverse pulse as possible without interfering with the next driven code pulse. The dropping away of relay ZTPB terminates the pulse by the opening of contact 53 in an obvious manner.

At the beginning of the next driven code pulse, the shifting of contact 25 causes the dropping away of relay ZTPA to open contact 54 in the circuit for the primary Winding 35 of the inverse code transmitter transformer ZTFR. The closure of back contact 5! of relay ZTPA reestablishes a circuit to cause relay ZTPB to be picked up again in a manner which has been described.

The presence of a train in track section 2T causes the dropping away of relay 2H, and renders the relays ZTPA and ZTPB inactive by the opening of contacts 52 and 50 respectively.

The restoration of the magnetic stick relay is made efiective at the beginning of the next driven code pulse because of the energization of a circuit including the winding of relay 3GP upon the closure of the coding contact 1800, assuming the trackway to be unoccupied by a train. Such circuit extends from including the coding contact [800, front contact 56 of relay 3H,

Winding of relay 3GP and lower winding of relay' 3AR to The two windings of relay 3AR are connected in opposition so that the energization of this Winding causes the restoration of the contacts 5| and 42 of relay 3AR back to the posi-,

tion from which they were actuated upon the reception of an inverse code pulse. The picking up of relay 3GP at the beginning of a driven code pulse opens the circuit for the upper winding of relay 3AR at back contact 55 to prevent energy in the track circuit from feeding back into the relay SAR during the transmission of such pulse.

It is to be understood that the inclusion of adjustable resistances at various points in the circuit is provided particularly as a means of adjustment for difierent ballast conditions, different lengths of track circuits, etc. that may be en countered in practice, rather than being otherwise essential.

Having thus described specifically the circuit organization provided for the transmission of driven and inverse codes according to the present invention through the track section 2T as a typi cal track section, it is believed to be readily apparent to those skilled in the art how a system can be established including several track sections to provide a signalling system for any given;

The back' stretch of track signalled for traffic in both directions. It is believed that it'will be readily apparent, from the description as set forth as to the mode of operation in a typical track section how the system would operate under various trafiic conditions that would be encountered in practice where a plurality of track sections are involved.

Having described a coded track circuit for a particular stretch of track as one specific embodiment of the present invention, it is desired to be understood that this form is selected to facilitate in the disclosure of the invention rather than to limit the number of forms which the invention may assume, and it is to be further understood that various adaptations, alterations and modifications may be applied to the specific form shown to meet the requirements in practice, Without in any manner departing from the spirit or scope of the present invention, except as limited by the appended claims.

What I claim is:

1. In a coded track circuit for railroads, a track section, an inverse code transmitter and a track relay connected to the track rails at one end of the track section, a magnetic stick relay at the other end of .the track section, a source of alternating current and a half-wave rectifier, driven code transmitting means at said other end of the track section for connecting said source of alternating current to the track rails of said track section through said rectifier and a winding of said magnetic stick relay connected in multiple, and circuit means for energizing said magnetic stick relay in polar opposition to said energization by said driven code transmitting means inresponse to each inverse code pulse transmitted through the track rails to that end of the track section.

2. In a coded track circuit for a railway track section, an inverse code transmitter and a track relay connected to the track rails at one end of the track section, a driven code transmitter at the other end of said track section, a source of alternating current and a half-wave rectifier, circuit means for connecting said source of alternating current to said track rails through said half-wave rectifier and a contact of said driven code transmitter, a magnetic stick relay having two windings, and circuit means for connecting i one of the windings of said magnetic stick relay across said half-wave rectifier and the other of said windings in polar opposition to said winding and across the track rails, whereby said magnetic stick relay is actuated to one position by inverse code pulses transmitted through the track rails and to the other position by the opposite half of the alternating current wave from the half used for the transmission of driven codes.

3. In a coded track circuit for a railway track section, an inverse code transmitter connected to the track rails at one end of the track section, a magnetic stick relay at the other end of the track section, a source of alternating current and a half-wave rectifi r, driven code transmitting means at said other end for transmitting driven code pulses through the track section, said means being effective to apply said source of alternating current to the track rails through said. half-wave rectifier for one-half of the Wave and through said magnetic stick relay for the other half or" the wave, a track relay connected across he track rails at said one end of said track section responsive only to a polarity of energization corresponding to the polarity ap- I 10 plied to the track rails through said half-wave rectifier, and circuit means for rendering said magnetic stick relay distinctively responsive to inverse code pulses in the track rails as compared to its energization in accordance with the transmission of driven code pulses.

4. In a coded track circuit for a railway track section, an inverse code transmitter and a track relay connected to the track rails at one end of the track section, a magnetic stick relay at the other end of the track section, a code transmitter relay at said other end of the track section, circuit means for energizing said code transmitter relay, said circuit means including coding contacts and a winding of said magnetic stick relay connected in series with the Winding of said code transmitter relay, circuit means for energizing said magnetic stick relay over the track rails with the opposite polarity to that provided locally by said circuit means for energizing said code transmitter relay to thereby cause said magnetic stick relay to be operated in response to pulses applied to the track rails by said inverse code transmitter, and circuit means for transmitting driven code pulses at said other end of the track section in accordance with the pulsing of said code transmitter relay.

5. In a coded track circuit for railroads, a track section, a step-down transformer at each end of said track section, a source of alternating current connected to the primary winding of said step-down transformer at each end of said track section, rectifying circuit means for directly and permanently connecting the secondary winding of said transformer at each end of said track section through a rectifier across the track rails at that end, the connections for the opposite ends of said track section being made of opposite polarities, a polarized relay at each .end of said track section, circuit means for connecting said polarized relay at each end across the track rails at that end so as to render said relay distinctively responsive only to the particular pclarity of energization applied to the rails at the opposite end of the track section, and code transmitting means at one end of the track section effective to form pulses for transmission over the track rails at a predetermined rate by intermittently interruptim said source of alternating current applied to the primary winding of said transformer at that end of the track section, code transmitting means at the other end being effective to transmit a pulse subsequent to the termination of each pulse transmitted through the track rails to that end of the track section by interrupting said source of alternating current applied to the primary winding of said transformer at that end of the track section.

6. In a coded track circuit for railroads, a

.track section, a step-down transformer at one end of said track section, a source of alternating current connected to the primary side of said transformer at said one end of said track section, rectifying circuit means permanently connecting the secondary side of said transformer through a rectifier across the track rails at said one end, coding contacts for interrupting the connection of said source of alternating current to said primary side of said transformer at a predetermined rate, an inverse code transmitter at the other end of said track section including a transformer and a rectifier with the secondary side oi said transformer permanently connected through the rectifier across the track rails in opposite polar relationship to the transformer andrectifier connection to the rails at said one end, said inverse code transmitter being effective to apply alternating current to the primary be distinctively responsive to the polarity of the pulses transmitted from the opposite end of said track section but unresponsive to the polarity of the pulses applied to the track rails at that end,

4 and circuit means for rendering active said inverse code transmitter following each operation of the polarized relay at the corresponding end of said section. V

7. In a coded track circuit for a section of railway track, a source of alternating current at each end of the sectionfa code transmitter at each end of the section and each having contacts,

a step-down transformer located at each end of the track section and each having primary andv secondary windings, circuit means at each end of the section including a rectifier for permanently connecting the secondary winding of the associated transformer across the track rails at its end of the track section, said connections at opposite ends being of opposite polarity, circuit means at one end of the track section including contacts of the associated transmitter for intermittently connecting the source of alternating current at that end to the primary winding of the associated transformer, an approach relay at said one end connected so as to be responsive only to pulses in the track rails between successive energizations of said primary winding, a polarized track relay at the other end of said 1 tively low potential transmitted from the opposite end of the section as a result of the control of relatively higher alternating current potentials by said transmitter contacts.

8. In a coded track circuit for a section of railway track, a step-down transformer at each end of said section and each having primary and secondary windings, a rectifier at each end of said section, circuit means at each end of said section permanently connecting the secondary winding of the transformer at that end in series with its associated rectifier across the track rails of said section, said connections at opposite ends of said section being of opposite polarity, a polarized ,code following track relay at each end of the section, a driven code transmitter at one end of said section intermittently operated at selected code rates and having a contact for applying Number alternating current energy to the primary-wind.- ing of its associated transformer to thereby transmit driven code pulses of direct current over said track section, circuit means at said one end for connecting the polarized track relay for its end across the track rails through a contact of said code transmitter which is opened when a code pulse is being transmitted, circuit means at the other end of said section permanently connecting said polarized track relay at that end directly across the track rails with such a polarity as to be responsive only to pulses received from the opposite end of the section, and inverse code transmitting means at such other end of the section rendered effective momentarily after the reception of each driven code pulse by said polarized track relay for momentarily applying alternating current to the secondary winding of said transformer at that end to thereby effect the transmission of an inverse code pulse for actuating the polarized code following track relay at said one end following each successive driven code pulse.

9. Code transmitting and receiving apparatus at one end of a track section comprising in combination, a step-down transformer having primary and secondary windings, a rectifier, circuit means permanently connecting said secondary winding in series with said rectifier across the rails of said section, a code following track relay responsive to only one polarity of energization, circuit means permanently connecting said relay across the trackrails of said section with a polarity .to be non-responsive to the output of said transformer through said rectifier, a source of alternating current, a transmitter relay having a contact of hard contact material for controlling relatively high potentials compared to the potentials applied to the track rails, said contact being operated in accordance with the code to be transmitted, and other circuit means including said contact at times effective to connect said alternating current source to said secondary winding of said transformer.

WADE H. REICHARD.

REFERENCES CITED The following references are of record in the file of this patent:

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